大乘起信論義疏 2.39

2.39

問曰：真妄其性各異，云何妄修而得熏真？良以真妄不相離，故妄染則染，妄淨則淨，故以妄修熏成真德。

Question: The natures of reality and falsity are distinct; how can false cultivation perfume the real? [Answer:] Truly, it is because reality and falsity are not separate from each other: thus when the false is defiled, it is defiled; when the false is pure, it is pure. Therefore, through false cultivation, true virtues are perfumed into completion.